**4. Smart targeting nanoparticles in antimicrobial photodynamic therapy**

Microbial infectious diseases, especially those due to bacterial and fungal infections, initially affect specific areas and may subsequently spread throughout the entire organism [64]. Therefore, in order to arrest bacterial pathogenesis, treatment modalities that identify and target affected areas, sites, and cells are preferred. The importance of using smart targeting NPs in aPDT emanates from the desire to direct such treatment to disease-affected areas, sites, and cells, with minimal or no negative effects on normal host tissue cells [65]. When the disease-infected site is external, and the aPDT treatment is topical rather than systemic, the purpose of smart NP-mediated targeting is to enhance selectivity for the infective microbial cells and their supporting EPS matrix over normal host tissue cells. In cases of deep tissue or systemic infection, however, such targeting has a general purpose of selectivity for disease cells [66]. Several methods have been reported to enhance selectivity for the disease cells over normal host tissue cells.
